In optical transmission, high-level quadrature amplitude modulation (QAM) is an attractive modulation format for achieving high spectral efficiency. To double spectral efficiency, polarization-division multiplexing (PDM) is often applied. There are several ways to transmit a PDM-QAM signal. One approach is to use two Cartesian or I/Q modulators, each driven by two multi-level drive signals, which can be generated by a pair of digital-to-analog converters (DACs). FIG. 1 shows a PDM-16QAM transmitter based on DACs. Each DAC generates a multi-level drive signal in accordance with two or more data signals provided thereto. This approach, however, has several drawbacks: (1) complex drive circuitry including multiple DACs are needed; (2) the I/Q modulators need to operate in a linear regime, so the modulation loss is high; and (3) the drive signals need to be amplified by linear RF amplifiers before driving the I/Q modulators, thereby increasing the requirements on the RF amplifiers and reducing the efficiency of the amplifiers.
Another approach to generate a PDM-QAM signal with n constellation points (PDM-n-QAM) is to use log2(n) I/Q modulators, each driven by a binary drive signal. As an example, four I/Q modulators are needed to generate a PDM-16QAM signal. Examples of this approach can be found in U.S. Pat. No. 7,558,487, entitled “Multilevel amplitude and phase encoded signal generation”, by Xiang Liu and Xing Wei, incorporated herein by reference in its entirety. With the use of binary drive signals, the aforementioned drawbacks of the DAC-based approach are avoided in this second approach. On the other hand, the use of DACs as in the first approach makes it possible to readily generate pilot symbols at different constellation points not limited to those of an n-QAM constellation. By thus using DACs, pilot symbol sequences with uniform amplitudes in both the time and frequency domains can be readily generated. Pilot symbol sequences with such properties are desirable for accurately probing the optical channel response, leading to good channel equalization.